1. Field of the Invention
The present invention relates to electrical contact elements for electrical devices, and more particularly to lithographic type, microelectronic spring contacts with improved contours.
2. Description of Related Art
Recent technological advances, such as described in U.S. Pat. No. 5,917,707 to Khandros et al., have provided small, flexible and resilient microelectronic spring contacts for mounting directly to substrates, such as semiconductor chips. The '707 patent discloses microelectronic spring contacts that are made using a wire bonding process that involves bonding a very fine wire to a substrate, and subsequent electro-plating of the wire to form a resilient element. These microelectronic contacts have provided substantial advantages in applications such as back-end wafer processing, and particularly for use as contact structures for probe cards, where they have replaced fine tungsten wires. It is further recognized, as described, for example, in U.S. Pat. Nos. 6,032,356 and 5,983,493 to Eldridge et al, that such substrate-mounted, microelectronic spring contacts can offer substantial advantages for making electrical connections between semiconductor devices in general, and in particular, for the purpose of performing wafer-level test and burn-in processes. Indeed, fine-pitch spring contacts offer potential advantages for any application where arrays of reliable electronic connectors are required, including for making both temporary and permanent electrical connections in almost every type of electronic device.
In practice, however, the cost of fabricating fine-pitch spring contacts has limited their range of applicability to less cost-sensitive applications. Much of the fabrication cost is associated with manufacturing equipment and process time. Contacts as described in the aforementioned patents are fabricated in a serial process (i.e., one at a time) that can not be readily converted into a parallel, many-at-a-time process. Thus, new types of contact structures, referred to herein as lithographic type microelectronic spring (or contact, or spring contact) structures, have been developed, using lithographic manufacturing processes that are well suited for producing multiple spring structures in parallel, thereby greatly reducing the cost associated with each contact. Exemplary lithographic type spring contacts, and processes for making them, are described in the commonly owned, co-pending U.S. Patent Applications “LITHOGRAPHICALLY DEFINED MICROELECTRONIC CONTACT STRUCTURES, Ser. No. 09/032,473 filed Feb. 26, 1998 by Pedersen and Khandros, and “MICROELECTRONIC CONTACT STRUCTURES”, Ser. No. 60/073,679, filed Feb. 4, 1998 by Pedersen and Khandros, both of which are incorporated herein by reference.
Lithographic type microelectronic spring contacts are subject to different design considerations than the plated and bonded wire microelectronic contacts currently in use, because of the characteristics of the lithographic manufacturing process. For example, lithographic type contacts are typically much smaller than wire-type microelectronic contacts, and tend to have characteristic cross-sections of relatively low-aspect ratio (i.e., flat) shape, in contrast to the circular or elliptical cross-sections typical for wire contacts. Because of their typical structural shape, lithographic type springs with essentially flat, rectangular cross-sections typically have relatively low stiffness (spring rates) and relatively small elastic ranges (that is, they may be deflected for only a short distance before becoming permanently deformed). Consequently, it is difficult to achieve the desired contact force needed to make a reliable electrical contact at the contact tip, without exceeding the elastic range of the spring, and thereby potentially damaging it.
Additionally, lithographically-defined contacts typically have a proportionally small “z-component,” that is, they may extend away from the substrate in a perpendicular (“z”) direction proportionally less than wire-type microelectronic contacts. This also limits the elastic range of the spring, because the contact force is typically applied in the z direction. One approach for providing adequate z-extension, for example, as disclosed in the above-referenced U.S. patent application Ser. Nos. 09/032,473 and 60/073,679, is to fabricate the spring structures using a series of lithographic steps, thereby building up the z-component extension with several lithographic layers. However, the use of multiple layers adds undesirable cost and complexity to the manufacturing process. Layered structures are also subject to undesirable stress concentration and stress corrosion cracking, because of the discontinuities (i.e., stepped structures) that result from layering processes.
Microelectronic spring structures are preferably provided with ample z-extension to permit mounting components, such as capacitors, below the structure. Adequate z-extension, together with adequate elastic range, is also desirable for reducing the amount of vertical positioning precision needed to make an electrical connection using the spring structure. Adequate stiffness is desired to ensure that the tip of the contact is applied to its electrical contact pad with sufficient force to ensure that a reliable electrical connection is made. Finally, improved strength and crack resistance is desirable for increasing the reliability and service life of the spring structure. The typical characteristics of very small feature size, relatively flat rectangular cross-section, and less z-component extension in proportion to spring length, make it very difficult to fabricate lithographically defined spring structures with adequate strength, stiffness, elastic range, and z-extension, to serve as reliable microelectronic spring contact structures.
A need therefore exists for an improved, lithographic type, microelectronic spring structure with improved spring characteristics, such as improved strength, stiffness, resistance to stress concentration cracking, and elastic range. A need further exists for an improved lithographic type microelectronic spring structure that can be fabricated in a single layer, thereby eliminating process layering steps and the associated costs. Furthermore, a need exists for an improved lithographic type microelectronic spring structure with greater z-component extension.